1. The Field of the Invention
The present invention generally relates to stator-driven devices, such as x-ray tubes. In particular, the present invention relates to an improved stator assembly that simplifies installation, reduces related vibration, and provides enhanced stator heat dissipation.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
In a typical x-ray device, x-rays are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure portion of an x-ray tube. Disposed within the evacuated enclosure is a cathode including an electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating disk that is mounted to a rotor shaft and bearing assembly. The evacuated enclosure is typically contained within an outer housing. Depending on the type of x-ray tube involved, the outer housing can be air-cooled or can contain a fluid, such as dielectric oil, to cool the x-ray tube.
In operation, an electric current is supplied to the electron source of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The target surface of the anode is oriented so that at least some of the x-rays are emitted through x-ray transmissive windows defined in the evacuated enclosure and the outer housing. The emitted x-ray beam can then be used for a variety of purposes, including materials analysis and medical evaluation/treatment.
Generally, only a small portion of the energy carried by the electrons striking the target surface of the anode is converted to x-rays. The majority of the energy is instead released as heat. To help dissipate this heat, many x-ray tubes employ rotary anodes, as already mentioned. Rotary anodes include a target surface on a circular face that is directly opposed to the electron source. An annular focal track comprising high-Z materials is formed on the target surface. During operation, the anode is spun at high speeds, thereby causing successive portions of the focal track to continuously rotate in and out of the focal spot of the electron beam emitted by the electron source. The heating caused by the impinging electrons is thus spread out over a relatively large area of the target surface and the underlying anode.
To enable its rotation during tube operation, the rotary anode is rotatably attached to a rotor assembly that is secured within the evacuated enclosure. The rotor assembly typically includes a rotor shaft that supports the anode, a bearing assembly, and a rotor disposed circumferentially about the bearing assembly. Correspondingly, a stator is circumferentially disposed about the rotor. During tube operation, the stator imparts a rotational force to the rotor of the rotor assembly, which in turn causes the anode to spin, as described above.
Various challenges exist with respect to the use of the stators in electrical devices, such as the x-ray tube described above. One challenge involves the manner in which the stator is mounted within the x-ray tube. Typically, a stator is affixed within the outer housing of the x-ray tube using mechanical fasteners, such as screws and brackets. While this method of attachment enables stator operation, it nevertheless suffers from various drawbacks. First, mechanical fastening of the stator to the outer housing creates a structure that is susceptible to mechanical vibration. Such vibration can be detrimental to the x-ray tube and its components. Additionally, vibration within the stator can cause acoustic resonance in the stator windings that results in undesired noise during tube operation. This noise can increase discomfort and stress for a patient, for example, when the x-ray tube is used in a medical imaging device.
Another problem that arises in connection with fastener-secured stators relates to the buildup of excessive heat. The process of anode rotation causes the stator to produce a substantial quantity of heat during tube operation. It is necessary for this heat to be continually removed in order to ensure adequate stator operation. This heat is typically removed from the stator using one of several means, depending upon the tube design. For instance, air-cooled tubes use air convection to remove excess heat from the stator. In oil-filled tubes, by contrast, a cooling fluid is circulated within the outer housing to remove heat from the stator. These attempted solutions alone, however, do little to alleviate the vibrational and acoustic challenges discussed above.
In an attempt to overcome some of the problems outlined above, some x-ray tubes employ a potting material that is packed around the stator after it has been positioned and secured within the outer housing of the x-ray tube by mechanical fasteners. Not only does packing potting about the stator require a significant expenditure of time during tube manufacture, but it also represents a time-consuming task should the replacement or repair of the stator ever become necessary. Additionally, the use of packed potting presents a potential contamination source for the cooling fluid in oil-filled tubes, which can result in both diminished effectiveness of the fluid and reduced operational lifetime of the x-ray tube.
Further, x-ray tubes having stators with packed potting also suffer from a reduced ability to remove heat from the stator given the low contact pressure existing between the potting material about the stator and the inner surface of the outer housing to which heat can be conducted from the stator.
In light of the above, a need exists for a stator-driven device, such as an x-ray tube, that overcomes the above stator-related challenges. In particular, there is a need for an x-ray tube featuring a stator assembly that is securely positioned within the x-ray tube such that acoustic noise and vibrational affects are substantially reduced or eliminated. Further, any such solution should be designed to effectively dissipate heat from the stator during tube operation. Additionally, any solution should feature a simple design to facilitate ease of assembly and stator change-out within the tube.